1. Field of the Invention
The invention relates to a method for the controlled growth of crystal whiskers and its application to the making of tip type microcathodes. More particularly, the invention relates to the field of the controlled growth of filamentary monocrystals or whiskers. By means of a known method (the VLS or vapor-liquid-solid method) developed in the sixties, it makes it possible to obtain regular matrices of whiskers with controlled height and diameter.
One of the possible applications is the making of matrices of microtips for field-effect cathodes.
2. Description of the Prior Art
Whiskers are filamentary monocrystals having a length/diameter ratio of more than 10. They have been known for a very long time, but interest in this type of crystals was stimulated at the beginning of the fifties when Herring and Gatt, in the Physical Review 85, (1952), 1060, showed that these crystals have mechanical properties similar to those of monocrystals.
E. I. Givargizov has also provided a complete description of this subject in E. Kaldis ed., Current Topics in Materials Science, Volume 1, North Holland, 1978, p. 79.
Two mechanisms have been put forward to explain filamentary growth in largely anisotropic vapor phase. These are, firstly, a mechanism that postulates the existence of a helicoidal dislocation at the center of the crystal, said dislocation behaving like an unsaturable atomic process (growth of the crystal helicoidally around the axis of the dislocation) and, secondly, a mechanism based on the VLS method wherein there is provision for a liquid phase at the top of the whisker as an essential source of anisotropy.
The principle of the VLS method is shown in FIGS. 1a and 1b for a silicon substrate (111). A layer of liquid, in which Si is soluble, is interposed between the vapor phase and the solid. Since the surface of liquid has a very large number of adsorption sites (each surface atom of the liquid is potentially an adsorption site) that are unsaturable (since there is immediate dissolving of the adsorbate and renewal of the site), this surface of liquid becomes a favored place of deposition which is at the origin of the anisotropy of growth.
The dissolved adsorbate prompts an over-saturation of the liquid which then expels solid matter to the liquid/solid interface, so as to return to equilibrium. The drop of liquid rises little by little on the expelled crystal which takes the form of a whisker with a diameter equal to that of the drop of liquid.
FIGS. 1a and 1b represent the VLS principle applied to the silicon but, quite clearly, it is possible to use a whole variety of different substrates associated with drops of liquid that are also highly varied (see for example Givargizov, already cited).
According to the known techniques, filamentary monocrystals or whiskers can therefore be made to grow on a substrate. However, this growth is not controlled and the points of growth are located randomly and unevenly on the surface of the substrate.
The present invention relates to a method for the controlled growth of whiskers using the VLS method. In controlling this growth, the invention therefore enables the making of whiskers at chosen points of the surface of the substrate and the making of matrices of filaments for example.